BETHESDA, Md – . From her perch as head of the National Institute on Drug Abuse, Nora Volkow watches anxiously as the country embarks on what she sees as a risky social experiment in legalizing marijuana.

For those who argue that marijuana is no more dangerous than tobacco and alcohol, Volkow has two main answers: We don't entirely know, and, simultaneously, that is precisely the point.

"Look at the evidence," Volkow said in an interview on the National Institutes of Health campus here, pointing to the harms already inflicted by tobacco and alcohol. "It's not subtle – it's huge. Legal drugs are the main problem that we have in our country as it relates to morbidity and mortality. By far. Many more people die of tobacco than all of the drugs together. Many more people die of alcohol than all of the illicit drugs together.

"And it's not because they are more dangerous or addictive. Not at all – they are less dangerous. It's because they are legal. … The legalization process generates a much greater exposure of people and hence of negative consequences that will emerge. And that's why I always say, 'Can we as a country afford to have a third legal drug? Can we?' We know the costs already on health care, we know the costs on accidents, on lost productivity. I let the numbers speak for themselves."

Volkow, 58, speaks rapidly, even urgently, in an accent that lingers from her childhood in Mexico. The great-granddaughter of Soviet communist Leon Trotsky, Volkow grew up in the Mexico City home where Trotsky was fatally attacked. It is easy to imagine, in her passionate determination, some of her ancestor's revolutionary fervor, melded with a scientist's evidentiary rigor.

As Colorado and Washington state approve the sale of marijuana for recreational use and other states consider following suit, Volkow says, the notion that legalization represents a modest, cost-free move is dangerously overblown. The evidence on the supposed safety of marijuana – particularly marijuana in its modern, far-more potent form – is far from clear enough to take this leap.

"I think that what we are seeing is a little bit of wishful thinking in the sense that we want to have a drug that will make us all feel good and believe that there are no harmful consequences," she said. "When you are intoxicated, your memory and learning are going to go down. When you are intoxicated, your motor coordination is going to go down. When you are repeatedly using marijuana, there is an increased risk for addiction. And if you are an adolescent and you are taking marijuana, there is a higher increased risk for addiction and there is also a higher risk for long-lasting decreases in cognitive capacity -- that is, lowering of IQ."

Adolescents are a chief focus of Volkow's worry, to the extent that when I observe that tobacco use is clearly worse for teens, she challenges that easy assumption.

"Wait a second. … Nicotine does not interfere with cognitive ability. So if you are an adolescent and you are smoking marijuana and going to school, it's going to interfere with your capacity to learn. So what is worse, as an adolescent right now? To have basically something that is jeopardizing your development educationally or to smoke a cigarette that when you are 60 years of age is going to lead to impaired pulmonary function and perhaps cancer? … I would argue that you do not want to mess with your cognitive capacity, that that is a very large price to pay."

Legalization advocates counter with two contradictory arguments: that marijuana is already readily available to teenagers who want it, and that the new laws impose strict controls on sales to minors. Volkow is unconvinced, arguing that the evidence from alcohol suggests that the already large number of teenagers who have tried marijuana by the time they graduate from high school – nearly half, according to the Centers for Disease Control and Prevention – will only increase, along with the already rising number of those who use it on a daily basis.

"Our kids are sensitive to norms, so if they feel that marijuana is harmful, their consumption goes down," Volkow said. Legalization sends the opposite message.

Vokow herself has never smoked pot – or, as she tends to say, "taken marijuana" – and she isn't tempted now that it is legally available in some places. "I'm not going to negate that I am curious," she said. "But I am terrified about doing anything that would interfere with my cognitive capacity. … I don't like to contaminate my perception of the world. I have too much respect for my brain."

Ruth Marcus is a columnist for the WashingtonPost, email marcusr@washpost.com